In a human body or an animal body, the most notable flexible tubes are blood vessels and airways. The airways may comprise trachea, main bronchi, small bronchiole, etc. In the medical diagnostic area, it is useful to know the response of a tube to an excitation. The excitation can be a natural excitation, such as a contraction of a heart, or an artificial excitation. The artificial excitation can be a pressure excitation from an external body. The response of a tube to an excitation can be used for diagnosis of pathologies or used for optimizing therapeutic treatment.
The response of a tube to an excitation can result in resonance. Resonance occurs when an external excitation leads to maximum energy storage in a tube and causes the maximal amplitude of the movements of the tube wall. The resonance of a tube relates to the movements caused by the excitation applied to a tube. The frequency at which resonance occurs is dependent, among other factors, on material properties of the tube. For diagnosis purposes, measuring the movement of a tube caused by an excitation is already practiced.
For example, arterial stiffness measurements are often based on measuring the time delay of a pressure pulse travelling from a position to another position along the artery. The time delay depends on the velocity of a pressure pulse travelling in the artery, and the time delay is correlated to the elasticity of the artery wall. Currently, the method of detecting the resonance frequency of an artery is to take out a segment of an artery of an animal, to apply a pressure pulse to excite the segment of the artery, and to measure the amplitude of the tube wall movements, which leads to an observation of resonance. When people notice that the segment of the artery is resonant, the frequency corresponding to the resonance is a resonance frequency of the artery. However, the current method is not useful for clinical practice, since the arteries are in the human body or animal body, so in normal clinical practice, people cannot see whether the arteries are resonant, which makes it difficult to determine a resonance frequency for the arteries based on the current method.
Another example is to measure the properties of airways, e.g. trachea, main bronchi, or small bronchioles, so as to measure the resonance frequency of airways for assisting cough by optimizing the treatment frequency when oscillation/percussion treatment is used, or to aid in diagnosis or disease management. A resonance frequency is an optimal oscillation frequency to help a patient to enhance mucus expectoration.
Currently, Forced Oscillation Technology (FOT) and Impulse Oscillometry (IOS) are used to measure air pressure and airflow at a mouth in response to periodical pressure variations and pressure pulses, respectively, caused by pressure applied to a lung system, in order to diagnose pathologies. Based on FOT and IOS, a resonance frequency is determined when there is no phase delay between the applied pressure pulse and the detected response at the mouth. However, the resonance frequency determined by FOT or IOS is limited to low frequencies due to the inherent inaccuracy of the system.
Furthermore, to improve lung mucus expectoration, normally, patients are instructed to cough in different ways, so as to first move mucus from the smaller bronchioles to the main bronchi, then to move mucus from the main bronchi to the trachea, and finally to cough mucus up. Thus, it is necessary to detect resonances for the smaller bronchioles, the main bronchi, and the trachea separately. Especially, some diseases only have an impact on part of the whole lung system. For example, COPD (Chronic Obstructive Pulmonary Disease) mainly has an effect on the small bronchioles, which shows a necessity to identify independently the resonance of the smaller airways of a lung system for diagnosing COPD. But, currently, based on FOT or IOS, it is difficult to distinguish resonances of the small bronchioles, the main bronchi, and trachea of a lung system separately.